Superphosphoric acid (SPA) is a condensation product of orthophosphoric acid. The minimum P.sub.2 O.sub.5 content of SPA is about 65% by weight, the maximum is greater than 100%. The P.sub.2 O.sub.5 content of most commercial grades of SPA is from 72% to about 85%.
Wet process phosphoric acid can be converted to SPA by means of conventional evaporation and dehydration techniques. SPA formed from unpurified wet process phosphoric acid is usually unstable due to ionic metallic impurities present therein. The impurities cause an increase in the viscosity of the SPA to such an extent that a semi-solid or glass-like material results. Further processing of such materials is extremely difficult.
On the other hand, purified wet process phosphoric acid yields a low viscosity SPA which can be readily handled (e.g. see U.S. Pat. Nos. 3,044,851 and 3,192,013). Williams et al, U.S. Pat. No. 3,694,153 and U.S. Pat. No. 4,053,564 of Bradford et al, removed ionic metallic impurities from phosphoric acid employing water immiscible orgaic sulfonic acids. Using solvent extraction techniques, wet process phosphoric acid was purified by removing the ionic metallic impurities therefrom.
While removing the impurities, P.sub.2 O.sub.5 values are entrained or coextracted with the metallic impurities into the organic phase. These P.sub.2 O.sub.5 values are usually lost when the water immiscible organic sulfonic acid is regenerated with a mineral acid because the mineral acid regeneration solution containing the ionic metallic impurities and the P.sub.2 O.sub.5 values is sent to waste disposal.
U.S. Pat. No. 4,082,836 of Ore recovers P.sub.2 O.sub.5 values in the organic extract by contacting the organic extractant with a wash phase comprising water or dilute phosphoric acid to form a loaded wash phase enriched with P.sub.2 O.sub.5 values extracted from the organic extractant. The loaded wash phase is then separated from the organic extractant phase. The washed organic extractant can then be regenerated on treatment with a mineral acid and be recycled into the process. In the Ore' process (and in those of U.S. Pat. No. 3,694,153 and 4,053,564) the sulfuric acid consumption to regenerate the organic extractant is about 10 times stoichiometric for removal of metal ions. Also, the waste acid must be neutralized (as with lime) before disposal. In the process of the present invention, sulfuric acid and lime concentration is much less than that half that in these prior art processes.
Aqueous phosphoric acid phase denotes a solution of phosphoric acid containing from about 1% to about 55% P.sub.2 O.sub.5 by weight which will be, is being, or has been subjected to solvent extraction. Terms such as phosphoric acid phase, crude phosphoric acid feed, unpurified phosphoric acid solution and purified phosphoric acid can be used to indicate an aqueous phosphoric acid phase.
Organic extractant phase denotes a solution in which at least one organic sulfonic acid is dissolved in an organic solvent and preferably containing a water immiscible organic phosphate or phosphonate and can also contain a water immiscible aliphatic alcohol. Extractant phase, organic phase, barren organic phase, loaded organic extractant phase, and regenerated organic extractant phase have been used to denote the organic extractant phase at some step in the solvent extraction process.
Wash phase denotes the aqueous phase (water or a dilute solution of phosphoric acid containing from about 10 grams/liter to about 300 grams/liter of P.sub.2 O.sub.5) which is used to remove P.sub.2 O.sub.5 values from the organic extractant phase; said P.sub.2 O.sub.5 values being coextracted with metal impurities into the organic extractant phase during the extracting step of solvent extraction process. Terms used in denote the wash phase are dilute aqueous phosphoric acid, dilute phosphoric acid, dilute phosphoric acid phase, dilute phosphoric acid stripping phase, water wash phase, loaded wash phase, water wash liquor enriched phosphoric acid solution and enriched phorphoric acid phase.
The mineral acid stripping phase is the solution of an inorganic acid such as sulfuric acid, nitric acid or hydrochloric acid which is used to regenerate the organic sulfonic acid present in the organic extractant phase. Sulfuric acid stripping solution, sulfuric acid stripping phase, fresh sulfuric acid stripping solution, stripping agent, sulfuric acid phase, waste sulfuric acid stripping, waste sulfuric acid stripping solution, waste solution and waste stripping agent are used to denote the mineral acid stripping phase.
A mineral acid is an inorganic acid havig an ionization constant equal to 10.sup.-3 or greater.